Monday, August 19, 2013

Into the Great Unknown: Passing Through the Permian Period

When we pushed the rafts off the beach at Lee's Ferry, my normal sense of time disappeared. For my journey into the Great Unknown, I didn't have a watch, and my smartphone was safely ensconced in one of the vehicles. My laptop never made it beyond Phoenix. Time took on a new meaning.

There was river time. That one is controlled by the sun, mostly. There was twilight and dawn, which guided when one went to bed, or arose. There was star time, the sweep of the Milky Way that told you the stage of the nighttime you had awakened in. There was worst sun time, the baking hot part of the day that often guided the choice of lunch spots. And there was afternoon shadows time, that told us it was time to pick a campsite.

Then there was geology time. As we descended through the first 80 miles of our river journey, we weren't just following the flow of the river downhill, but were also making our way back in geologic time. As the canyon deepened (or as the cliffs rose higher around us), we were encountering a sequence of successively older and older rocks. The youngest rocks, about 250 million years old, were exposed at Lee's Ferry, and at Mile 80 we would encounter some of the oldest rocks in the canyon, at 1.7-1.8 billion years.

As we started down the river, there were still small waterfalls tumbling off the cliffs, courtesy of the just-ended cloudburst. I was especially entranced by the double fall at Mile 4, which was trickling over ledges of the Kaibab and Toroweap formations. The Kaibab, which is familiar to all visitors at Grand Canyon National Park, forms the rim of the Grand Canyon. It is a sandy limestone that formed in a tropical shallow marine environment that developed at a time when the supercontinent Pangea had come together. The Toroweap Formation is broadly similar to the Kaibab, but is a limey sandstone. The difference might not mean much to non-geologists, but it means much to geologists who are interpreting the environment in which the rock formed. Where the Kaibab formed in offshore shoals and reefs, the Toroweap formed along the sandy coastline. Both formations are highly variable and the Kaibab especially has a rich fossil record, including corals, bryozoans, sponges, clams, gastropods (snails), and the occasional shark tooth.

Within a few million years, 90% or more of the species found as fossils in these rocks (and all other rocks worldwide) would be gone in the greatest extinction event of all geologic history. This event did not involve the dinosaurs; instead, it quite possibly made it possible for the dinosaurs to evolve and rise to dominance in terrestrial environments.

We would see the Kaibab and Toroweap formations at river level for less than 4 miles. They would be visible at times during the rest of the trip, but only as remote cliffs as much as a mile above us. At Mile 4, under another trickling waterfall (below), we crossed the contact between the Toroweap Formation, and dropped into exposures of the Coconino Sandstone. We passed beneath the Navajo Bridges at Mile 4.5, and I knew suddenly that we were indeed beyond all parts of the Colorado River with which I had any familiarity.

The Coconino Sandstone is an extraordinary rock unit. It has a distinctive pattern of crossbedding, a sort of diagonal layer formed as sand cascaded down the slipface of a sand dune. The crossbedding can be seen above just over the river vegetation. It formed in early
Permian time as sand dune “sea” in a desert that extended across northern
Arizona, New Mexico and southern Utah and extending north as far as present-day
Montana. The Coconino
preserves more than two dozen kinds of tracks, from large amphibians or
reptiles to scorpions or spiders. We didn't see any of them because the Coconino was exposed along the river for only about a mile, and we weren't ready to stop yet.

By Mile 5, we had reached the contact of the Coconino with the underlying Hermit Formation (or Hermit Shale as I learned it). The Hermit is a bright red-brown formation composed primarily of silt and shale that formed in the coastal floodplains of an extensive river system. The rivers were coming from somewhere geologically special, but I'll have to save that for our next post. The Hermit was just the tail end of an important geological event in the region during late Paleozoic time.

People can be forgiven for thinking that the Colorado River carved the Grand Canyon. That's true in one sense, but a river can generally only cut downwards, and it cannot account of the vast breadth of the tributaries and side canyons. Once a steep gorge has been carved, instabilities are introduced (i.e. oversteepening of slopes) that leads to mass wasting, in other words, landsliding. Much of the canyon has been produced by a combination of landslides and debris flows/flash floods. We passed numerous examples in the first days of the trip, including the spectacular rockfall consisting of Kaibab, Toroweap and Coconino rocks in the picture below (to the right).

The storm damage was catching up to us as well. Not a tree was visible from Lee's Ferry or on the surrounding cliffs, but as the day progressed, the river became nearly choked with branches, cones and seeds from juniper, pinyon and other trees that grew at the upper end of the Paria River and other tributaries. We didn't have to bail the self-bailing rafts, but I often had to grab handfuls of debris from the boat after getting splashed in a few rapids.

A bit further down the river we saw another recent rockfall...

And at the ten mile mark, we passed the imaginatively named Tenmile Rock, a huge slab of Coconino Sandstone that broke away from the cliff, tumbled down the slope, and landed in the river without quite falling over flat.

It was huge!

The rapids the first day were an introduction to a pattern that would continue throughout the trip. The river flowed placidly through the canyon until we reached a tributary drainage. Debris flows emanating from these side canyons burst into the main channel, partly damming the channel, and pushing the main flow of the Colorado River to one side or another. With the constriction, the river would speed up, and smooth undulations would develop in the top of the rapid, forming a "vee" pointing downstream. The vee was bounded by lateral waves, and at some point the rower would either punch through the laterals (especially in the biggest rapids), or follow through the roller coaster of big waves in the center of the channel.

A series of eddies would form at the base of the rapid (literally a large whirlpool flowing partly upstream), allowing the first boats down a chance to take position in anticipation of assisting anyone ejected from a boat in the midst of the rapid.

The seriously big rapids had special problems, including pour-overs, holes, and exposed boulders that needed to be avoided at all costs. When approaching such rapids, the rowers would pull out, and climb to a vantage point that offered a chance to scout the rapid. A discussion of the best possible run would ensue, and the bigger the rapid, the longer the discussion (and for the rookies, increasing apprehension). An order was picked for the boats and off we would go.

Our first day included two riffles, Paria and Brown, rated 1 or 2 out of 10 on the Colorado Rapids classification system. We also were baptized by two class 5 rapids, Badger and Soap Creek. One thing became abundantly clear: don't take a Colorado River rafting trip if you hate getting wet. Literally every rapid would find a way to douse the passengers in the front of the raft. Mind you, when the temperatures were high, it was a welcome relief, and I generally looked forward to getting soaked! Even the smallest rapids could surprise; we found out later on that a boat flipped in the Paria Riffles a day or two after we came through.

You will also notice that I have few pictures of the rapids. My camera and silty, muddy water don't mix well, so when I heard the roar of a rapid coming up, I would snap a shot or two of rafts going over the edge, and when we got safely to the bottom I would pull the camera out and try to snap a shot or two looking back upstream, such as in the picture above at Soap Creek Rapid.

Our first day, we managed (after the late start) a modest 12.4 miles. We stayed that night at 12.4 Mile Camp (where do they come up with these creative names?). It was a cool evening because of the cloud cover. We knew hotter days would come, so we enjoyed the moderate temperatures. I set up camp, and after a dinner of fresh shrimp and linguini, I hit the sack.

A wonderfully easy to start your rafting - no watch/phone/technology. However, that Class 5 rapid must have been very exciting! I like to stick to no more than a Class 4. Thank you for the beautiful photos and explanations - a pleasure for this armchair traveller.

About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.